The present invention relates to a liquid level measuring system for measuring a residual quantity of a liquid such as a fuel stored in a tank.
The vehicle that uses a gasoline as the fuel to drive is equipped with a tank that stores the fuel. A fuel gauge used to check a remaining fuel in the tank all the time (referred to as a “liquid level gauge” hereinafter) is mounted on a dashboard. This liquid level gauge is operated based on an output of a resistance type liquid level sensor (referred simply to as a “resistance type sensor” hereinafter) that has a simple structure and is available inexpensively (see Patent Literature 1, for example).
FIG. 6 is a block diagram showing a liquid level measuring system including such resistance type sensor and the liquid level gauge in the prior art. This liquid level measuring system is constructed by connecting a measuring device main body 12 to a resistance type sensor 11 via a cord, or the like. The measuring device main body 12 consists of an input circuit 13, a central processing unit (abbreviated as “CPU” hereinafter) 14, and a liquid level gauge 15.
Details of the resistance type sensor 11 and the input circuit 13 are given as shown in FIG. 7. The resistance type sensor 11 consists of a moving contact that is operated together with a float arm that is floating on the liquid surface in the tank, for example, and a resistor that outputs a resistance value in response to its contact position when the moving contact moves and contacts the resistor, i.e., outputs a change of the voltage. In FIG. 7, the resistance type sensor 11 is illustrated in the form of a variable resistance.
The input circuit 13 functions to convert a change of the resistance value of the resistance type sensor 11 into the change of the output voltage. This input circuit 13 consists of a pull-up resistor 16 provided on the power supply side, a voltage smoothing capacitor 17, and resistors 18, 19 in combination.
The CPU 14 converts/processes an analog output voltage of the input circuit 13 into a digital pulse that corresponds to a swing angle of an indicator of the liquid level gauge 15. This process of converting the voltage into the digital pulse responding to the swing angle of the indicator in the CPU 14 is carried out by referring to a data table that is prepared in advance in a memory, or the like.
This data table has a format shown in FIG. 8, for example. A capacity (residual quantity) and a swing angle of the indicator in regarding to the resistance value of the variable resistor 16 and the input voltage value of the CPU 14 are listed in a tabular form every sender F, meter F, and meter 3/4.
Therefore, in this liquid level measuring system, as shown in FIG. 9A, the input circuit 13 outputs an analogue voltage in response to a change of the resistance value of the variable resistor as the resistance type sensor 11. As shown in FIG. 9B, this analogue voltage is in proportion to the change of the resistance value. As shown in FIG. 9C, the CPU 14 converts this analogue voltage into a digital pulse to operate the indicator of the liquid level gauge 15. As a result, the liquid level gauge 15 is able to indicate the liquid level.
Meanwhile, in the liquid level measuring system using such resistance type sensor, in some cases the correct measured results cannot be attained owing to the defective contact between the moving contact and the resistor, the earth (GND) floating, the connector contact failure, or the like.
For this reason, the liquid level measuring system of the non-contact and digital pulse output type, which does not have a contact mechanism consisting of the moving contact and the resistor, has been proposed in the prior art.
FIG. 10 is a block diagram showing a liquid level measuring system of the non-contact and digital pulse output type in the prior art. This liquid level measuring system is constructed by connecting a measuring device main body 21 to a non-contact type sensor 20 via a cord, or the like. The measuring device main body 21 includes an input circuit 22, a CPU 23, and a liquid level gauge 24.
Details of the non-contact type sensor 20 and the input circuit 22 are given as shown in FIG. 11. Out of these elements, the non-contact type sensor 20, although not shown, is composed of a Hall IC and a magnet that is turned (moved) around this Hall IC. This magnet is fitted to an L-shaped arm 26 that has a float 25 at its top end.
Therefore, in this non-contact type sensor 20, a movement of the float 25 that moves vertically in response to the liquid level in the tank is transferred to the magnet via the L-shaped arm 26. The Hall IC functions to output a digital pulse a duty factor of which can respond to the movement (amount of turn) of the magnet.
This non-contact type sensor 20 senses a change of the liquid level as a magnetic change, then processes this sensed signal in a digital (PWM) fashion, and then outputs the resultant signal.
The input circuit 22 consists of a load resistor 27 used to extract a digital pulse voltage from a current output of the Hall IC constituting the non-contact type sensor 20, voltage-dividing resistors 28, 29 for generating a reference voltage from the power supply voltage, and a voltage-level converting portion 30 for comparing an output voltage of the Hall IC with the reference voltage. A pull-up resistor 31 is connected to the input side of the CPU 23.
The CPU 23 serves to convert the digital pulse output from the input circuit 22 into a digital pulse that responds to a swing angle of the indicator in the liquid level gauge 24. The digital pulse converting process is carried out by the CPU 23 in compliance with the data table that is prepared previously in the memory, or the like.
This data table has a format shown in FIG. 13, for example. The capacity and the swing angle of the indicator in regarding to the duty factor of the digital pulse input into the CPU are listed in a tabular form.
Therefore, in this liquid level measuring system, as shown in FIG. 12A, the Hall IC outputs a digital pulse current based on an output of the Hall element. Then, as shown in FIG. 12B, this digital pulse current is converted into the digital pulse voltage by using the load resistor 27. At that time, a level of the digital pulse voltage is set near a predetermined threshold voltage level (2.4 V).
Then, the digital pulse voltage that is subjected to the level adjustment is input into the voltage-level converting portion 30, and is compared with a reference voltage there (FIG. 12C). Then, as shown in FIG. 12D, only the digital pulse voltage that is in excess of this reference voltage is amplified up to a predetermined voltage level by the operation of the CPU 23, and then is transferred to the CPU 23.
The CPU 23 accepts the digital pulse voltage and operates the indicator of the liquid level gauge 24. As a result, the liquid level gauge 24 is able to indicate the liquid level. Patent Literature 1: JP-A-2001-171366.
However, in order to operate the liquid level gauge 15 based on the digital pulse in the liquid level measuring system having the resistance type sensor 11 in the prior art, (a) the input circuit 13 of the measuring device main body 12 shown in FIG. 7 must be changed to the complicated input circuit 22 in the measuring device main body 21 shown in FIG. 11, and (b) a software (the data table in FIG. 8) of the CPU 14 must be changed to a software (the data table in FIG. 13) of the CPU 23.
Such massive change is required of the hardware and the software, and thus such change is a serious obstacle to newly load the hardware and the software in the existing vehicle or load the hardware and the software in the newly developed vehicle. As a result, such a problem existed that the above obstacle constitutes a factor that delays quality improvement (non-contact measuring approach) of the liquid level measuring system.